Back to EveryPatent.com
| United States Patent |
6,017,425
|
|
Park
, et al.
|
January 25, 2000
|
Photocatalyst, preparation therefor and method for producing hydrogen
using the same
Abstract
The present invention relates to a photocatalyst for producing hydrogen,
harmless to the environment and by which a large quantity of hydrogen is
efficiently produced at low temperatures without using any organic
promoter, represented by the following formula:
Cs(a)X(c)/T(b)
wherein "a" represents a percentage by weight of impregnated Cs on the
basis of the weight of a carrier, being limited up to 6.0; "X" is a
promoter selected from Ni, Co and Fe; "c" represents a percentage by
weight of the promoter on the basis of the total weight of Cs and the
promoter, being limited up to 50.0; "T" is the carrier consisting of a
zinc sulfide mixture comprising an inorganic compound and zinc sulfide
with the molar ratio of zinc:sulfur ranging from 1:0.1 to 1:2.8; and "b"
represents a percentage by weight of the inorganic compound on the basis
of the total amount of the ZnS mixture, being limited up to 50. The
present invention also relates to a method for preparing the photocatalyst
and to a method for producing hydrogen using the photocatalyst.
| Inventors:
|
Park; Dae Chul (Daejon, KR);
Lim; Sang Yun (Daejon, KR)
|
| Assignee:
|
Korea Research Institute of Technology (Daejon, KR)
|
| Appl. No.:
|
029829 |
| Filed:
|
March 10, 1998 |
| PCT Filed:
|
September 17, 1996
|
| PCT NO:
|
PCT/KR96/00161
|
| 371 Date:
|
March 10, 1998
|
| 102(e) Date:
|
March 10, 1998
|
| PCT PUB.NO.:
|
WO97/12668 |
| PCT PUB. Date:
|
April 10, 1997 |
Foreign Application Priority Data
| Current U.S. Class: |
204/157.52; 419/27; 502/216; 502/222; 502/242; 502/243; 502/245; 502/246; 502/326; 502/330; 502/338; 502/339; 502/344 |
| Intern'l Class: |
C01B 003/00; B01J 027/02; B22F 003/26 |
| Field of Search: |
204/157.52
502/330,326,344,216,218,222,242,243,245,246,338,339
419/27
|
References Cited
| Foreign Patent Documents |
| 96/06675 | Mar., 1996 | WO.
| |
Other References
Derwent Abstract AN 95-166550 [22] of JP 07 088 380, Apr. 1995.
Patent Abstracts of Japan, vol. 11, No. 262 (C-442), of JP 62-65 743, Mar.
1987.
Borodenko et al., "Kinetics of Adsorption of Cesium Vapors on Zinc", Zh.
Fiz. Khim., vol. 41, No. 3, pp. 545-549. Abstract Only, 1967, no month
available.
Borodenko et al., "Exchange Adsorption of Cesium and Hydrogen or Carbon
Monoxide on Zinc Sulfide", Zh. Fiz. Khim., vol. 43, No. 7, pp. 1854-1855.
Abstract Only, 1969, no month available.
|
Primary Examiner: Wong; Edna
Attorney, Agent or Firm: Jacobson, Price, Holman & Stern, PLLC
Parent Case Text
This is a national stage application of PCT/KR96/00161 filed Sep. 17, 1996.
Claims
What is claimed is:
1. A photocatalyst for producing hydrogen, comprising the following general
formula:
Cs(a)X(c)/T(b)
wherein "a" represents a percentage by weight of impregnated Cs on the
basis of the weight of a carrier, being present in the amount of up to
6.0; "X" is a promoter selected from the group consisting of Ni, Co and
Fe; "c" represents a percentage by weight of the promoter on the basis of
the total weight of Cs and the promoter, being present in the amount of up
to 50.0; "T" is the carrier comprising an inorganic compound and zinc
sulfide having a molar ratio of zinc:sulfur ranging from 1:0.1 to 1:2.8;
and "b" represents a percentage by weight of the inorganic compound on the
basis of the total amount of said carrier, being up to 50.
2. The photocatalyst in accordance with claim 1, wherein said inorganic
compound is selected from the group consisting of alumina, silica,
niobate, titania and zirconia.
3. The photocatalyst in accordance with claim 1, wherein the inorganic
compound is present in the carrier.
4. The photocatalyst in accordance with claim 1, wherein cesium is
impregnated in the carrier.
5. The photocatalyst in accordance with claim 1, wherein the promoter is
impregnated in the carrier.
6. The photocatalyst in accordance with claim 1, wherein cesium and the
promoter are impregnated in the carrier.
7. A method for preparing a photocatalyst for hydrogen production,
comprising the steps of:
mixing zinc and sulfur in a molar ratio of 1:0.1-2.8;
sufficiently pulverizing the mixture with a mechanical means; sintering the
mixture at 200 to 700.degree. C. for 2 to 5 hours, to give a ZnS carrier;
and
impregnating cesium, an active ingredient in the carrier.
8. A method in accordance with claim 7, further comprising the step of
impregnating a promoter selected from the group consisting of Ni, Co and
Fe, following the impregnation of cesium.
9. The method in accordance with claim 7, wherein cesium is impregnated in
a form of an aqueous cesium carbonate solution.
10. The method in accordance with claim 7, further comprising the step of
adding an inorganic compound selected from the group consisting of
alumina, silica, niobate, titania and zirconia to the ZnS mixture at up to
50% by weight.
11. A method for producing hydrogen, in which a suspension of the
photocatalyst of claim 1 in water is subjected to photoreaction by
irradiating visible or ultraviolet light onto the suspension with stirring
in a photoreactor.
12. The method in accordance with claim 11, wherein the photoreaction is
carried out at 10 to 85.degree. C. and at 0.1 to 5 atm.
Description
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates, in general, to a novel photocatalyst and,
more particularly, to a photoreaction in which hydrogen can be efficiently
and economically produced from water in the presence of the photocatalyst.
Also, the present invention is concerned with a method for preparing the
photocatalyst and a method for producing hydrogen.
2. Description of the Prior Art
Hydrogen is a very important material in the chemical industry. For
example, it is used to produce ammonia and to synthesize hydrogen
chloride. Also, it is an essential material for hydrogenation in which
unsaturated compounds are converted into saturated ones. In addition,
hydrogen plays a pivotal role in improving the quality of petroleum
products, that is, in the hydrotreating processes including hydrogen
addition, desulfurization, denitrogenation and demetallization. Another
example of the use of hydrogen is the contact hydrogenation of carbon
dioxide in which carbon dioxide, which causes the warmth of the globe, is
reclaimed, immobilized and reused. In addition, hydrogen is anticipated to
be a pollution-free, clear energy source substituting for the existing
fossil fuels.
Conventional techniques for obtaining hydrogen include extraction from
fossil fuels, such as naphtha, modification of natural gas, reaction of
vapor with iron, reaction of water with metal, electrolysis of water and
etc. These techniques are economically unfavorable because immense heat or
electric energy is required. The conventional techniques have another
disadvantage of generating a large quantity of by-products, carbon dioxide
or oxygen (upon electrolysis). As mentioned above, carbon dioxide is a
serious air pollutant causing the greenhouse effect of the globe. When
oxygen is generated, the hydrogen is difficult to separate from the
oxygen, owing to which the process becomes complicated. Such by-products
could make it difficult to obtain high purity hydrogen in high yields.
Since hydrogen production using such conventional techniques is usually
carried out at relatively high temperatures, most of the relating
equipment, e.g. reactors and purifying apparatuses, are designed to be
resistant to heat and thus, they are very expensive, which is an
economically unfavorable factor.
Hydrogen gas can readily escape from the gravity of the earth because it is
of low specific gravity and most of it exists in water or inorganic forms.
For these reasons, only a small quantity of hydrogen exists in the
atmosphere. Therefore, the development of the techniques whereby a high
purity of hydrogen can be efficiently obtained from water is very
important in that the urgent problem of exploiting substitute energy
sources can be solved and the material for the chemical industry can be
secured.
Recently the techniques for producing hydrogen from water have been
developed in which a photocatalyst is used to decompose water into
hydrogen and oxygen. However, there are few published prior arts relating
to the photocatalyst for producing hydrogen, the representatives of which
are exemplified by Japanese Pat. Laid-Open Publication Nos. Sho.
62-191045, Sho. 63-107815 and Hei. 1-208301.
Japanese Pat. Laid-Open Publication No. Sho. 62-191045 discloses that
hydrogen is generated from an aqueous Na.sub.2 S solution in the presence
of a rare-earth element compound as a photo-catalyst by a photolysis
reaction. The rare-earth element compound has an advantage of exhibiting
an optical catalytic activity in the range of the visible light.
Japanese Pat. Laid-Open Publication No. Sho. 63-107815 describes a
photolysis reaction in which a composite oxide of niobium and alkali earth
metal is used as a photocatalyst, to generate hydrogen from a methanol
solution in water. Similarly, this photocatalyst has an advantage of being
active in the range of the visible light.
However, these above prior arts are disadvantageous in that the amount of
the hydrogen generated is very small.
In Japanese Pat. Laid-Open Publication No. Hei. 1-208301, water and
aluminum are subjected to a thermal reaction, to generate hydrogen. This
has an advantage of being high in efficiency of hydrogen-generating but a
significant disadvantage of requiring immense thermal energy because the
thermal reaction occurs only at 600.degree. C. or higher.
Korean Pat. Appl'n. No. 95-7721, which is believed to solve the above
problems to some degree, by the present inventor, suggests a photocatalyst
represented by the following general formula I:
Cs(a)/K.sub.4 Nb.sub.6 O.sub.17 I
In the presence of the photocatalyst, ultraviolet light is irradiated onto
an aqueous solution mixed with oxygen-containing organic compounds, such
as formaldehyde and alcohol, acting as a hydrogen-generating promoter, to
produce hydrogen from water.
This technique has little affect on the environment and can generate
hydrogen at low temperatures, e.g. room temperature. However, in spite of
using the oxygen-containing organic compounds as a hydrogen-generating
promoter, hydrogen is produced at unsatisfactory amounts.
SUMMARY OF THE INVENTION
It is an objective of the present invention to overcome the above problems
encountered in prior arts and to provide a novel photocatalyst for
producing hydrogen, which is harmless to the environment and by which a
large quantity of hydrogen is efficiently produced at low temperatures,
e.g. room temperature, without using any organic promoter.
It is another objective of the present invention to provide a method for
preparing the photocatalyst.
It is a further objective of the present invention to provide a method for
economically producing hydrogen using the photocatalyst.
There has been significant and intensive research and development directed
to the definition and manufacture of a photocatalyst for hydrogen
production for a wide variety of commercial applications by the present
inventor. In accordance with the research and development, the present
invention is based on the finding that a ZnS carrier comprising ZnS (molar
ratio Zn:S=1:0.1-2.8) alone or in combination with inorganic compound
selected from the group consisting of silica, alumina, niobate, titania
and zirconia has a powerful ability to generate hydrogen in high
efficiency when in contact with primary or secondary distilled water or
simply pretreated water at 10 to 85.degree. C. and at 0.1 to 5 atm. If
necessary, cesium, acting as an inorganic active ingredient, and a
promoter selected from the group consisting of Ni, Co and Fe can be
impregnated in the carrier.
In accordance with an aspect of the present invention, there is provided a
photocatalyst represented by the following general formula II:
Cs(a)X(c)/T(b) II
wherein character "a" represents a percentage by weight of impregnated Cs
on the basis of the weight of a carrier, being limited up to 6.0;
character "X" is a promoter selected from Ni, Co and Fe and may be mixed
and impregnated following the impregnation of Cs; character "c" represents
a percentage by weight of the promoter on the basis of the total weight of
Cs and the promoter, being limited up to 50.0; character "T" is the
carrier consisting of a ZnS mixture of ZnS (molar ratio Zn:S=1:0.1-2.8)
and an inorganic compound selected from silica, alumina, niobate, titania
and zirconia; and character "b" represents a percentage by weight of the
inorganic compound on the basis of the total weight of the ZnS mixture,
being limited up to 50.
In accordance with another aspect of the present invention, there is
provided a method for preparing a photocatalyst, comprising the steps of:
mixing Zn and S in a molar ratio of 1:0.1-2.8, sufficiently pulverizing
the mixture with a mechanical means, such as a ball mill, sintering the
mixture at 200 to 700.degree. C. for 2 to 5 hours, to give a carrier, and
optionally impregnating an inorganic active ingredient (Cs) and/or a
promoter (X) in the carrier. When the carrier is made of ZnS and another
inorganic compound, such as silica, the inorganic compound is added to the
ZnS mixture at up to 50% by weight, prior to the mixing step.
In accordance with a further aspect of the present invention, there is
provided a method for producing hydrogen, in which ultraviolet or visible
light is irradiated onto water in the presence of the photocatalyst at 10
to 85.degree. C. and at 0.1 to 5 atm, to induce the photolysis of water to
generate hydrogen gas.
DETAILED DESCRIPTION OF THE INVENTION
The present invention pertains to a photocatalyst which is superior in
producing hydrogen. According to the present invention, it is obtained by
mixing zinc (Zn) and sulfur (S) at a predetermined molar ratio in a ball
mill, centrifuging the mixture at a speed of 100 to 130 rpm for 24 to 72
hours, and sintering it at 200 to 700.degree. C. for 2 to 5 hours. The
sintered body itself shows the catalytic activity in the photolysis of
water. In addition, it can serve as a carrier when impregnating an active
ingredient, such as cesium, and optionally a promoter, such as Ni, Fe and
Co.
As to the amount of zinc and sulfur, it is preferred that the molar ratio
of zinc to sulfur ranges from 1:0.1 to 1:2.8 and more preferably from
1:0.7 to 1:1.5. For example, if the molar ratio departs from the ranges,
the efficacy of the photocatalyst is very low.
The carrier can be prepared from the ZnS alone or in combination with
inorganic compounds. In the case of combination, inorganic compounds are
added at an amount of up to 50% by weight based on the total amount of the
carrier. For example, if the amount of the inorganic compounds exceeds 50%
by weight, the hydrogen generation capacity of the resulting photocatalyst
is significantly attenuated. Available inorganic compounds are selected
from the group consisting of silica, alumina, niobate, titania and
zirconia.
When the carrier is made from high purity ZnS alone, the resulting
photocatalyst is superior in catalytic activity, so that a large amount of
hydrogen can be obtained. However, such a carrier is very poor in light
fastness. In contrast, the carrier made from the ZnS mixture consisting of
the ZnS and the inorganic compounds has lower catalytic activity but is
significantly improved in light fastness.
As mentioned above, the photocatalyst of the present invention can be
obtained by impregnating a catalytically active inorganic ingredient in
the carrier, which also has catalytic activity. Cesium (Cs) is suitable
for the catalytically active inorganic ingredient and the resulting
photocatalyst is much more active than the carrier alone. The active
ingredient is preferably added at an amount of up to 6.0% by weight and
more preferably up to 3.5% by weight. For example, if too much active
ingredient is impregnated, the impregnation effect is remarkably lowered.
Optionally, promoter(s) may be impregnated at an amount of up to 50% by
weight based on the total weight of the catalytically active ingredient
and the promoter(s). Available are Ni, Co and Fe. When the active
ingredient (Cs) alone is used, the activity of the resulting photocatalyst
(the reactivity to light) decreases soon with the lapse of time. In
contrast, the combination of the active ingredient and the promoter allows
the photocatalyst to endure light for longer times.
In order to impregnate cesium in the carrier, cesium is first converted
into an aqueous cesium carbonate (Cs.sub.2 CO.sub.3) solution which is,
then, impregnated in the carrier, according to an ordinary impregnation
technique. Thereafter, the combined solution was stirred for 3 to 15 hours
at 65 to 90.degree. C. and dried. The resultant one may not be pulverized
further.
The photocatalyst thus obtained can be used to produce hydrogen. For this,
it is suspended in primary or secondary distilled water or simply
pretreated water and placed under ultraviolet or visible light in a
photoreactor, such as a closed, gas-circulating system, while stirring. In
this state, hydrogen is efficiently produced even though the
oxygen-containing organic compound promoter, such as formaldehyde and
alcohol, used in Korean Pat. Appl'n. No. 95-7721, a previous patent of the
present inventor, is not employed. The photoreaction is preferably carried
out at 10 to 85.degree. C. and more preferably at 15 to 35.degree. C. and
preferably at a pressure of 0.1 to 5 atm and more preferably at 1 atm.
A better understanding of the present invention may be obtained through the
following examples which are set forth to illustrate, but are not to be
construed as the limit of the present invention.
In the following examples, hydrogen productivity is defined as follows:
##EQU1##
PREPARATION EXAMPLE I
Preparation of ZnS Carrier
1.0 mol (58.9 g) of zinc and 0.1 mol (3.2 g) of sulfur were mixed together,
pulverized in a ball mill for 28 hours at 110 rpm and sintered for 2 hours
at 300.degree. C., to prepare a ZnS carrier.
PREPARATION EXAMPLES II THROUGH VI
The procedure of Preparation Example I was repeated using 0.2 mol (6.4 g),
1.0 mol (32.0 g), 1.4 mol (44.8 g), 2.6 mol (83.2 g) and 2.8 mol (89.6 g)
of sulfur, respectively instead of 0.1 mol (3.2 g) of sulfur.
PREPARATION EXAMPLE VII
Preparation of ZnS type Carrier
1.0 mol (58.9 g) of zinc, 1.4 mol (44.8 g) of sulfur and 44.44 g (30% by
weight) of silica were mixed together, pulverized in a ball mill for 26
hours at 115 rpm and sintered for 2 hours at 300.degree. C., to give a ZnS
type carrier.
PREPARATION EXAMPLES VIII THROUGH XI
The same procedure with that of Preparation Example VII was repeated using
alumina, niobate, titania and zirconia, respectively, instead of silica.
PREPARATION EXAMPLES XII AND XIII
Preparation of Photocatalyst containing Cs
Cs was converted into an aqueous Cs.sub.2 CO.sub.3 solution and impregnated
in the carriers obtained in Preparation Examples VII and VIII in such a
manner that the amount of the Cs might be 0.1% by weight based on the
total weight of the carrier by controlling the amount of the cesium
carbonate. Thereafter, the resulting solution was stirred at 70.degree. C.
for 5 hours and dried, to give photocatalysts.
PREPARATION EXAMPLES XIV THROUGH XVI
Preparation of Photocatalyst comprising Cs and X(Ni, Co, Fe)
3.0 g of the photocatalyst obtained in Preparation Example XII was immersed
in 250 ml of 0.01 M Co, Fe or Ni solution in water and the solution was
stirred at 25.degree. C. for 48 hours. The photocatalyst was washed with a
copious amount of distilled water and filtered. The excess metal salt
which might be on the surface of the catalyst was washed off so that the
impregnation amount of Co, Fe or Ni might be 0.05% by weight based on the
total weight of the carrier. Drying at 110.degree. C. for 24 hours and
sintering at 300.degree. C. for 2 hours resulted in a novel photocatalyst.
EXAMPLES I THROUGH XVI
Each of 1.0 g of the photocatalysts obtained in Preparation Examples I
through XVI was suspended in 500 ml of secondary distilled water and the
suspension was placed in a closed, gas-circulating photoreactor and
stirred at a speed of 400 rpm and then, irradiated by ultraviolet light
from a high pressure mercury lamp, to produce hydrogen whose amounts was
analyzed with gas chromatography. The results are given as shown in Table
1 below.
Before the start of the photoreaction, the air remaining in the closed,
gas-circulating photoreactor was removed by using a vacuum system and
argon gas was charged therein to an extent of 0.2 atm. The ultraviolet
light may increase the temperature of the photoreactor. This can be
prevented by circulating cooling water around the photoreactor. In the
present invention, the photoreactor was kept at 15 to 20.degree. C.
EXAMPLE XVII
Hydrogen was produced in the same manner as that of Example XII, except
that visible light was used instead of ultraviolet light. The amount of
hydrogen generated was analyzed and the result is given as shown in Table
1 below.
TABLE 1
______________________________________
Hydrogen Produced from Water in presence of Photocatalyst
Exmp. Catalyst Light Hydrogen (.mu.mol/hr)
______________________________________
I ZnS uv 17,510
Zn:S = 10:1 (mol)
II ZnS uv 17,542
Zn:S = 5:1 (mol)
III ZnS uv 28,540
Zn:S = 1:1 (mol)
IV ZnS uv 33,920
Zn:S = 1:1.4 (mol)
V ZnS uv 26,130
Zn:S = 1:2.6 (mol)
VI ZnS uv 24,930
Zn:S = 1:2.8 (mol)
VII ZnS, silica uv 19,600
Zn:S = 1:1.4 (mol)
ZnS:silica = 70:30 (wt %)
VIII ZnS, Alumina uv 17,520
Zn:S = 1:1.4 (mol)
ZnS:Alumina = 70:30 (wt %)
IX ZnS, Niobate uv 20,210
Zn:S = 1:1.4 (mol)
ZnS:Alumina = 70:30 (wt %)
X ZnS, Titania uv 28,900
Zn:S = 1:1.4 (mol)
ZnS:Titania = 70:30 (wt %)
XI ZnS, Zirconia uv 17,820
Zn:S 1:1.4 (mol)
ZnS:Zirconia = 70:30 (wt %)
XII Cs/ZnS, Silica uv 22,421
Zn:S = 1:1.4 (mol)
ZnS:Silica = 70:30 (wt %)
Impregnated Cs 0.1 wt %
XIII Cs/ZnS, Alumina uv 19,380
Zn:S = 1:1.4 (mol)
ZnS:Alumina = 70:30 (wt %)
Impregnated Cs 0.1 wt %
XIV Cs.Co/ZnS, Silica uv 24,320
Zn:S = 1:1.4 (mol)
ZnS:Silica = 70:30 (wt %)
Impregnated Cs 0.1 wt %
Co/(Cs + Co) = 33 wt %
XV Cs.Fe/ZnS, Silica uv 21,312
Zn:S = 1:1.4 (mol)
ZnS:Silica = 70:30 (wt %)
Impregnated Cs 0.1 wt %
Fe/(Cs + Fe) = 33 wt %
XVI Cs.Ni/ZnS, Silica uv 19,960
Zn:S = 1:1.4 (mol)
ZnS:Silica = 70:30 (wt %)
Impregnated Cs 0.1 wt %
Ni/(Cs + Ni) = 33 wt %
XVII Cs/ZnS, Silica visible 14,380
Zn:S = 1:1.4 (mol)
ZnS:Silica = 70:30 (wt %)
Impregnated Cs 0.1 wt %
______________________________________
Comparative Example I
A cesium carbonate solution in 10 ml of secondary distilled water was mixed
with 5 g of K.sub.4 Nb.sub.6 O.sub.17 carrier of lamellar structure in
such a manner that the impregnation amount of cesium might be 1.0% by
weight and the resulting solution was stirred overnight at 25.degree. C.
and dried in vacuo. Sintering at 200.degree. C. for 3 hours gave
Cs/K.sub.4 Nb.sub.6 O.sub.17 photocatalyst. The amount of the hydrogen
generated by using the photocatalyst was measured in the same manner as
that of Examples and was found to be 370.6 .mu.mol/hr.
Comparative Example II
The same procedure as that of Comparative Example I was repeated except
that, instead of secondary distilled water, 20% by volume of an aqueous
formaldehyde solution was used to increase the production amount of
hydrogen.
Gas chromatography showed that the amount of hydrogen produced is 37,445.0
.mu.mol/hr but, the reactants could not be reused owing to the addition of
the organic compound.
Comparative Example III
The same procedure as that of Example I was repeated except that the molar
ratio of Zn to S is 1:3. Gas chromatography showed that hydrogen was
produced at an amount of 26,980 .mu.mol/hr. However, when preparing the
photocatalyst, the excess sulfur was found to be ignited too much.
Comparative Example IV
The same procedure as that of Example VII was repeated except that the
weight ratio of ZnS to SiO.sub.2 was 30:70. Gas chromatography showed that
hydrogen was produced at an amount of 6,530 .mu.mol/hr.
The present invention has been described in an illustrative manner, and it
is to be understood the terminology used is intended to be in the nature
of description rather than of limitation.
Many modifications and variations of the present invention are possible in
light of the above teachings. Therefore, it is to be understood that
within the scope of the appended claims, the invention may be practiced
otherwise than as specifically described.
Top